Minimum temperature, DPD (the difference between ambient temperature and dew point), precipitation, and wind speed are the four major components that affect fog formation. The frequency of tule fog occurrence is proportional to the higher air pollution in California. Īs of 2014, it has been discovered that the quantity of tule fog has decreased in the Central Valley from when it was initially studied from 1981–1999 compared to 2001–2012. Depending on the region within the California Central Valley, the composition of tule fog can vary in element or ion concentrations. Furthermore, ammonia is the most commonly found single ion and usually is measured to be more than half of the measured ions in the fog. Such events can leave an invisible glaze of black ice on roadways, making travel especially treacherous.īesides water droplets, the composition of tule fog in the San Joaquin and Sacramento valleys includes ammonia, nitrate and sulfate concentrations. Episodes of freezing drizzle occasionally accompany tule fog events during winter. Sunlight cannot penetrate the fog layer, keeping temperatures below freezing. Tule fog events are often accompanied by drizzle. ![]() There were two fatalities and 39 injuries in the crash. Visibility was about 200 feet (60 m) at the time of the accident. On the morning of November 3, 2007, heavy tule fog caused a massive pile-up that included 108 passenger vehicles and 18 big-rig trucks on northbound State Route 99 between Fowler and Fresno. ![]() In February 2002, two people were killed in an 80-plus-car pile-up on State Route 99 between Kingsburg and Selma. It took 26 hours to clear away all the wreckage and reopen the freeway. ![]() In one such accident on Interstate 5 near Elk Grove south of Sacramento, 25 cars and nine big-rig trucks collided inside a fog bank on December 12, 1997. The variability in visibility is the cause of many chain-reaction pile-ups on roads and freeways. Visibility can vary rapidly in only a few feet, visibility can go from 10 feet (3.0 m) to near zero. Visibility in tule fog is usually less than an eighth of a mile (about 600 ft or 200 m). Tule fog usually remains longer in the southern and eastern parts of the Central Valley, because winter storms with strong winds and turbulent air affect the northern Central Valley more often. Daytime heating (cloud-penetrating visible light wavelengths transformed to infrared by the ground) sometimes evaporates the fog in patches, although the air remains chilly and hazy below the inversion and fog reforms soon after sunset. Once tule fog has formed, turbulent air is necessary to break through the temperature inversion layer. Above the cold, foggy layer, the air is typically mild, dry and clear. Tule fog is a low cloud, usually below 2,000 feet (600 m) in altitude and can be seen from above by driving up into the foothills of the Sierra Nevada to the east or the Coast Ranges to the west. Tule fog often contains light drizzle or freezing drizzle where temperatures are sufficiently cold. Because of the density of the cold air in the winter, winds are not able to dislodge the fog and the high pressure of the warmer air above the mountaintops presses down on the cold air trapped in the valley, resulting in a dense, immobile fog that can last for days or at times for weeks undisturbed. Tule fog is characteristically confined mainly to the Central Valley due to the mountain ranges surrounding it. Tule fog occasionally drifts as far west as the San Francisco Bay Area via the Carquinez Strait, and can even drift westward out through the Golden Gate, opposite to the usual course of the coastal fog. In California, tule fog can extend from Bakersfield to Red Bluff, covering a distance of over 650 kilometres (400 mi). The nights are longer in the winter months, which allows an extended period of ground cooling, and thereby a pronounced temperature inversion at a low altitude. ![]() Tule fog is a radiation fog, which condenses when there is a high relative humidity (typically after a heavy rain), calm winds, and rapid cooling during the night. Tule fog is the leading cause of weather-related accidents in California. This phenomenon is named after the tule grass wetlands ( tulares) of the Central Valley. The official time frame for tule fog to form is from November 1 to March 31. Tule fog forms from late fall through early spring (California's winter season) after the first significant rainfall. Tule fog ( / ˈ t uː l iː/) is a thick ground fog that settles in the San Joaquin Valley and Sacramento Valley areas of California's Central Valley. Annual fog event in California's Central Valley Tule fog captured by NASA's Terra Satellite Tule fog settled on an orchard in Stanislaus County in late December.
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